The Science Behind Prescribed Burning


 Fire Intervals and Biodiversity Responses in the South-West of WA

The possible negative impacts on biodiversity of regular fuel reduction burning are a frequent issue of concern raised by those opposed to the practice.  DEC has recently released the results of a major research project studying the effect of a range of fire intervals on biodiversity in jarrah forest and adjacent shrublands that conclusively shows that there is no detrimental effect.

The work is reported in Fire Note 64, published by the Bushfire Cooperative Research Centre. The full report can be viewed at

Researchers Wittkuhn and McCaw selected 30 sites in jarrah forest (a dry sclerophyll forest type) and seasonally flooded shrublands (known locally as “flats) in the Warren bioregion, which lies to the north of Walpole. The sites included a range of fire intervals (i.e., fuel age and frequency) within a recently burnt area covering both vegetation types.

The fire intervals were as follows:

* Short-short: two fires at about 5 year intervals

* Long-long; two fires at intervals of more than 10 years

* Mixed/moderate: a mixture of fire intervals ranging from 6-9 years

* Very long: a 30 year interval before the most recent fire

The species composition and abundance of ants, beetles and vascular plants were monitored at all sites. In addition, the forest sites were surveyed for macro-fungi and vertebrates.

Analysis of their data showed that the varying fire intervals in both vegetation types had no persistent effect on either richness or composition of the biota.

We know, from long experience, as well as research (e.g. Boer et al, 2009) that forest fuels less than 6 years old greatly mitigate the intensity of a bushfire and make suppression much easier and safer. This latest research shows that maintaining fuels in this way across the landscape does not have adverse effects on a broad range of biodiversity values.

This being so, there is no argument at all, on biodiversity grounds, that we should not aim to maintain the entire dry sclerophyll forest estate in WA in this condition. This applies to private forest as well as that which is publicly owned.


Boer, M.M.,R.J.Sadler, R.S Wittkuhn, L.McCaw and P.F.Grierson. (2009). Long-term impacts of prescribed burning on regional extent and incidence of wildfires – evidence from fifty years of active fire management in SW forests. Forest Ecology and Management 259:132-142.

Fire Behaviour and Fuel Reduction Burning Techniques

The fire research program in Western Australia commenced in 1961 has continued to the present day, albeit at a reduced level in the last 15-20 years. It has included a variety of forest types and covered many aspects of fire behaviour, fire ecology, nutrient recycling and fire ignition techniques. Some has been carried out in collaboration with CSIRO. Overall, it has produced a vast fund of information and understanding of fire management issues specific to Western Australian conditions.

Some research on fire impacts had been carried out earlier, focusing on possible effects on nutrient recycling. Initially, the new 1961 fire research program was oriented to providing answers to the pressing operational problems of the time – how to place the field fuel reduction burning (FRB) program on a scientific basis, where the hit and miss approach of the past could be replaced by systematic field procedures and where outcomes of burns could be predicted with confidence.

This required much basic work on litter drying rates and measuring the intensity and rate of spread of experimental fires under a range of fuel and weather conditions. Over 600 experimental fires were conducted in the jarrah and karri forest types, so that there is a very sound scientific basis for the practice of FRB in Western Australian forests.

A considerable amount of work was concerned with the development of methods for increasing the rate of progress of the FRB program. In cooperation with CSIRO, this led to the development of the unique aerial ignition system. Aerial burning enabled much larger areas to be burned under ideal weather conditions, improving the control of the burning process as well as reducing the cost.

What FRB does is turn this…..

into this!!

Obviously, this is a whole lot easier, and safer, to suppress. It also has less impact on the overstorey trees, which are critical to maintaining the forest environment. Such burns maintain forest health by recycling nutrients bound up in litter, twigs, etc. Smoke has been shown to stimulate the regeneration of many understorey plants.

Later, attention turned to the ecological impacts of fire on flora and fauna. Long term study sites comparing flora impacts of various degrees of fire intensity and season of burn with long unburnt areas were set up. Radio tracking was employed to study the behaviour of fauna during burns (see Christensen, 1980, reference given below). Because the biological diversity of forested ecosystems is complex, fire effects are similarly complex. A good summary of the current state of knowledge is given by Friend and Wayne (2003).

Since about 1980, additional fire ecology research has been carried out in academia, which has contributed to the large amount of information we now have. However, while there is always a great deal more detail we wish to know, we do have sufficient information to be sure that FRB in forested ecosystems does maintain biodiversity.

The main issues currently debated are concerned with how to integrate the needs of different components of the biota. Trying to eliminate fire from the forest is not a practical option, and has adverse effects on biodiversity, while large high intensity bushfires have a disastrous impact on biodiversity and really have the potential to cause a faunal species to become extinct.

Environmental Impacts on Flora and Fauna

Carefully planned burns can be used to create the habitats required by some animal species, or to protect those habitats from wildfire. A good example of the latter is the swampy vegetation required by the mainland quokka. In a spring burn, the dense swamp vegetation will not burn, but the surrounding higher and drier forest areas will burn, thus creating a firebreak around the quokka refuge areas.

The behaviour of forest animals during FRBs has been monitored using radio location equipment. It has been found that they are not particularly concerned by low intensity burns, and can easily evade the flames and move off to unburnt areas. In contrast, high intensity wildfires are commonly observed to incinerate many native animals.

Ground dwelling animals, such as kangaroos and wallabies, move into freshly burned areas within days of a burn, as many native plants rapidly produce new shoots that are keenly sought after as fodder. In fact, if only small areas are burnt at one time, the grazing pressure can be so intense as to cause excessive damage to the regrowth.

Many other ground dwelling animals, such as reptiles, take shelter under logs or in holes, so are protected from the heat, unless fire intensity is too high (as is found in a wildfire). Birds are generally also not particularly concerned by low intensity prescribed burns.

In the main forest zone there are no ground-nesting species that would lose nests in fire and those that nest in understorey shrubs have generally completed their breeding by the time that vegetation type will carry a burn. However, on some occasions there will be some losses of some species of fauna. This is not necessarily a serious problem, except in the case of rare and endangered species, as population numbers can be quickly restored from adjacent populations and from unburnt patches. Such ebbs and flows of populations are common in nature and represent one way of ensuring widespread gene flows. Rare species receive special consideration in the planning of the DEC burning program.

 Forest Fuels and Animals

A question that is sometimes asked is: Is it true that fuel only accumulates in forests these days because of the loss of small mammals that used to eat it or dig it into the soil?

This is an interesting question, as it has an appealing ‘balance of nature’ feel about it and it seems to offer a neat solution to the controversial question of the role of fire in our forest ecosystems. We instinctively feel therefore, that the answer to the question should be yes. Regrettably, the short answer, however, is no, returning the small animals to the forest is not the answer to the ’so called’ problem of fuel build up.

The idea appears to have arisen because of observations made by various people of native animals in natural bush enclosures. When animals are kept in a bush enclosure (a section of bushland fenced off, usually to keep out exotic predators) they tend to build up in numbers to unnaturally high populations that can cause unnatural effects. These effects are usually obvious to the naked eye and depending on the species and other factors they can include;

#  openness of the ground  layer – caused by trampling and compaction of the litter layer which also causes earlier breakdown of otherwise long persisting leaves and bark.

#  a high percentage of bare ground and disturbed soil – caused by excessive digging activity exposing the soil and also causing earlier breakdown of the litter layer by burying leaf litter.

#  openness of the undergrowth – caused by excessive grazing pressure on leaves of scrubs that under normal circumstances would be grazed only lightly or not at all.

#  general openness of the area – caused by lack of regeneration of grasses, herbs and shrubs due to over-grazing by the animals.

The result – a general impression of a lack of build up of fuel on the woodland or forest floor. This impression can be very strong since the phenomenon is ‘observed with the observers own eyes’ and often also re-enforces previously held convictions with respect to fuel build up in our forests. The majority of Australians of European extraction have never been comfortable with the concept of fire, especially frequent fire, being a natural part of the Australian forest ecosystem.

The reality is that small animals were never present in numbers that would cause the observed effects over the general forest area. In the jarrah forest, we know this because in areas where animals still exist in high numbers, for example, some of the forest area to the east of Manjimup, fuels still accumulate at rates that have allowed several wildfires to occur over the last few decades. In the karri forest where the understorey is dense, fuel accumulates rapidly in spite of the fact that the pre-European fauna is still largely intact.

In some open woodland communities where fuel accumulation is naturally slow and where mammal diversity was high small mammals would have had some effect on the rate of fuel accumulation. Also a few woodland species that occur naturally in high numbers in specific and restricted habitats, e.g., the tammar wallaby (Macropus eugenii), can have some effect on fuel accumulation rates.

The tammar wallaby reduces the fuel to a limited extent under the thickets in which it shelters, however, over the majority of the area that it inhabits, the tammar has limited impact on fuels. This observation is supported by the fact that wildfires in the Perup district, to the east of Manjimup, have been observed to burn out thickets with dense populations of tammar on at least two occasions over the last 30 years.

In conclusion, the effect of small mammals on fuel accumulation in forest areas is likely to have been insignificant and of little practical importance with respect to fire protection.

Useful References on Bush Fire Management

  • Underwood, R.J.,R,J, Sneeuwjagt and H.G.Styles. (1985) The Contribution of Prescribed Burning to Forest Fire Control in Western Australia: Case Studies. WAIT Environmental Studies Group Report No 14: 153-170. WA Institute of Technology.
  • Friend, G. and A Wayne.(2003). Relationships between mammals and fire in south-west Western Australia: what we know and what we need to know. P363-380 in Fire and ecosystems of south-west Western Australia: impacts and management. Ed by I. Abbott and N. Burrows..Backhuys Publishers.
  • Christensen, P.E.S. (1980). The Biology of Bettongia penicillata and Macropus eugenii in Relation to Fire. Bulletin 91, Forests Department of Western Australia.
  • Bradshaw, F.J, R.Adams, R.Sneeuwjagt, K.Low, J.J.Havel, J.R.Bartle and G.L.Stoneman. (1991). The Jarrah Forest: A Case Study in Multiple Use. Ch 1 in Forest Management in Australia. Ed By F.H McKinnell, E.R Hopkins and J.E.D.Fox. Surrey Beatty and Sons
  • Department of Conservation and Land Management, Western Australia.(2000) Fire: the force of life. Landscope Special Fire Edition.
  • Department of Conservation and Land Management, Western Australia (2005). Fire: the force of life. Landscope Special Fire Edition Volume 2.
  • Muller,C. (2001). Review of Fire Operations in Forest Regions Managed by the Department of Conservation and Land Management.
  • Abbott, I., P. van Heurck and L. Wong. (1984). Responses to Long Term Fire Exclusion: Physical, Chemical and Faunal Features of Litter and Soil in a Western Australian Forest. Aust. For. 47(4):237-242.
  • Wilson, A.A.G. and I.S. Ferguson. (1984). Fight or Flee? – A Case Study of the Mount Macedon Bushfire. Aust. For.47(4):230-236.
  • Burrows, N.D. (2003). Fire for Life.Landscope Winter 2003.
  • Burrows, N.D. (2008). Linking Fire Ecology and Fire management in South-West Australian Forest Landscapes. Forest Ecology and Management 255:2394-2406.
  • Jurskis, V.and R. Underwood. (2013). Human Fires and Wildfires on Sydney Sandstones: History Informs Management. Fire Ecology (9):8-24.